1. Field of the Invention
The invention relates to a process for isolating pure diketene with recovery of materials of value by continuous, two-stage distillation of crude diketene at reduced pressure and with continuous reaction of the resulting distillation residue with acetic acid.
2. Background Art
In the industrial dimerization of technical-grade ketene, for example as described in DE-A-2301655 (US-A-4001332), an 82 to 87% pure crude diketene is obtained, which, besides small amounts of dissolved ketene, acetone and acetic acid, is contaminated by a further 5 to 7% of acetic anhydride and 8 to 10% of oligomeric or polymeric ketenes, which can be described by the general formula (CH.sub.2 .dbd.C.dbd.O).sub.n+1, where n&gt;1. This crude diketene is unsuitable for use as starting material for numerous syntheses. Before further use, it must be purified, which can conveniently be achieved by distillation. If known methods are used for this, then account must be taken of product decomposition and polymerization, caused by the high reactivity of the diketene, which detract from the economics and the effectiveness of the purification process, impairing the quality of the pure diketene obtained. In addition, the distillation of the crude diketene is associated with a high safety risk, in particular because of the 4 to 5% of ketene trimer, which is very unstable, always present.
A particular processing and safety problem of the industrial distillation of crude diketene is the separation and further handling of the polymeric ketenes left over, which are obtained as a residue which, under standard conditions, is viscous to solid--depending on the residual content of diketene and acetic anhydride unstable and tends to decompose spontaneously. In practice, therefore, pure diketene is not distilled from the crude diketene quantitatively, but only in part, in general up to about 50%, based on the initial content, and the remaining crude diketene, now containing even more by-products, is passed on for further utilization with all the associated disadvantages. Furthermore, this unstable residue cannot be passed on to a reutilization procedure, but must be immediately and continually disposed of, for example by incineration, which gives a discernible loss of materials of value, based on ketene, and an undesirable dependence on a dedicated residue disposal system.
Attempts have already been made, in the dimerization of ketene, to reduce the proportion of polymeric ketenes by addition of the inhibitors, for example sulfur dioxide (EP-A 0377438=US-A 4999438). This does reduce the problem, but does not solve it. A residual content of higher polymeric ketenes amounting to from 8 to 10% always remains. In addition, such additives tend to reduce the stability of the crude diketene during the distillative purification, which is shown by lower distillation yields or increased formation of reactive residue.
The alternative solution, to isolate only some of the diketene in pure form from the crude diketene by distillation and to feed the remainder to suitable secondary reactions, for example the preparation of various acetoacetic esters, only solves the safety aspect of the problem. Apart from the disadvantages of coproduction, account must in this case be taken of other serious disadvantages in the secondary syntheses which lead to high consumption of materials, high manufacturing costs and product quality problems, because of the formation of numerous by-products which must be removed again from the desired product.
As disclosed in EP-A-0287894 (US-A-4808735), attempts have therefore been made to reduce the safety risk during the preparation and distillation of crude diketene by addition of compounds having functional hydroxyl groups, for , example water, alcohols or carboxylic acids, which react with the trimeric ketene to form thermally stable butanecarboxylic acid derivatives. Since, however, the diketene too undergoes analogous reactions with these additives, the desired degradation of triketene is only incomplete or a high excess of these additives must be used, which makes this measure impractical because of the side reactions with diketenes.
The same disadvantages are shown by the processes disclosed in CA-A-846162 and CA-A-850145, in which the crude diketene is reacted with water to remove acetic anhydride by-product and subsequently worked up by means of extraction and/or distillation.